Method of selecting carrier frequency for call origination

ABSTRACT

A mobile station in an idle state is associated with a current sector-carrier that has a current pilot channel at a current carrier frequency. In response to a request to originate a call, the mobile station compares its current sector-carrier with one or more candidate sector-carriers that have pilot channels at carrier frequencies distinct from the current carrier frequency. The mobile station measures received signal strengths of the current carrier frequency and the one or more candidate carrier frequencies and measures pilot signal levels of the current pilot channel and the one or more candidate pilot channels. If a candidate sector-carrier has a carrier frequency with a received signal strength that exceeds that of the current sector-carrier, and has a pilot channel with a pilot signal level that exceeds that of the current sector-carrier, the mobile station uses that candidate sector-carrier to transmit a call origination message.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 11/081,272, filed Mar. 16,2005, which application is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to wireless telecommunications and, moreparticularly, to a method of selecting a carrier frequency for a mobilestation to use when transmitting a call origination message.

2. Description of Related Art

Wireless telecommunications networks typically include a plurality ofdistinct wireless coverage areas, e.g., “cells” or “sectors,” withinwhich mobile stations can wirelessly communicate with the network. Insome cases, a base transceiver station (BTS) may provide a singlewireless coverage area, e.g., a cell. More typically, however, a BTSprovides multiple wireless coverage areas, e.g., by using directionalantennas, to provide multiple sectors within a given cell.

Each sector may have allocated to it a distinct set of downlink channelsthat the BTS uses for transmitting signals to mobile stations and adistinct set of uplink channels that mobile stations may use fortransmitting signals to the BTS. The downlink channels may include, forexample, a pilot channel, paging channels, and traffic channels. Theuplink channels may include, for example, access channels and trafficchannels. The uplink and downlink channels may use different carrierfrequencies. For example, in the case of code division multiple access(CDMA) wireless communication, the uplink channels for a given sectormay all use the same uplink carrier frequency and may be distinguishedby different codes. The downlink channels for the given sector may alluse the same downlink carrier frequency, i.e., a carrier frequency thatis different than that used for the uplink channels, and may bedistinguished by different codes.

Increasingly, however, wireless telecommunications networks are usingmultiple frequency plans in some or all of their sectors in order toprovide additional wireless capacity in those sectors. In particular, agiven sector may have allocated to it a plurality of uplink carrierfrequencies, each with associated uplink channels, and a plurality ofcorresponding downlink carrier frequencies, each with associateddownlink channels. As a result, it is often useful to consider thewireless coverage provided by a network as being divided into aplurality of distinct “sector-carriers.” Each “sector-carrier” isassociated with a particular geographic area, e.g., a cell or sector,and with a particular set of one or more carrier frequencies. Forexample, a sector-carrier may have a set of uplink channels that use aparticular uplink carrier frequency and a set of downlink channels thatuse a particular downlink carrier frequency.

The number of different carrier frequencies that are available in agiven sector may depend on the configuration of the BTS that providesthe wireless coverage in that sector. For example, a BTS may beconfigured to communicate using two different uplink carrier frequenciesand two different downlink carrier frequencies. In that case, eachsector that is provided by that BTS may be associated with twosector-carriers: (i) one that uses a first uplink carrier frequency anda first downlink carrier frequency; and (ii) one that uses a seconduplink carrier frequency and a second downlink carrier frequency.Moreover, these same carrier frequencies may be used by different BTSsin the network. Thus, two different sector-carriers may be distinguishedon the basis of their geographic locations (e.g., they may be associatedwith different sectors) and/or on the basis of their carrier frequencies(e.g., they may be associated with the same sector but use differentcarrier frequencies).

A mobile station may be associated with a particular sector-carrier byusing or monitoring one or more of its channels. For example, a mobilestation that is engaged in a call may be associated with a particularsector-carrier by using an uplink and/or downlink traffic channel ofthat sector-carrier. When a mobile station is not engaged in a call,e.g., in an idle state, the mobile station may be associated with aparticular sector-carrier by monitoring the paging channel of thatsector-carrier for messages from the network. The sector-carrier withwhich a mobile station is associated may change. For example, a handofffrom one sector-carrier to another may occur when the mobile stationchanges its location. Such handoffs may occur when the mobile station isin an idle state, or when the mobile station is engaged in a call.

Once associated with a particular sector-carrier, a mobile stationtypically uses the access channels of that sector-carrier to transmitmessages to the network. For example, to originate a call, the mobilestation would typically transmit a call origination message in an accesschannel of the mobile station's current sector-carrier, i.e., thesector-carrier with which the mobile station is associated in the idlestate. Then, if the network receives and is able to validate the callorigination message, the network may transmit a channel assignmentmessage to the mobile station via a paging channel of thatsector-carrier. However, if multiple carrier frequencies are availableat that location, the network may also instruct the mobile station tobegin using a different sector-carrier, such as a sector-carrier thatuses different uplink and downlink carrier frequencies than thesector-carrier that the mobile station used to transmit the callorigination message. For example, in response to a mobile station's callorigination message, the network may transmit an extended channelassignment message (ECAM), instructing the mobile station to use adifferent sector-carrier, with different carrier frequencies, for thecall.

As noted above, networks often provide multiple carrier frequencies incertain areas in order to increase the number of traffic channelsavailable, and, thus, increase the network's capacity in that area.However, in order to realize the benefits of using multiple carrierfrequencies when a mobile station tries to originate a call, the networkmust receive the call origination message in the originalsector-carrier's access channel and the mobile station must receive thechannel assignment message that the network transmits in the originalsector-carrier's paging channel. If the original sector-carrier's RFconditions are inadequate, however, then the messages needed tooriginate the call may not be received. For example, the mobile stationmay be located in an area with low signal strength from the BTS, or themobile station may be located in an area with high interference. If, asa result of such poor RF conditions, the network fails to receive themobile station's call origination message or the mobile station fails toreceive the network's channel assignment message, the mobile station'sattempt to originate the call may be unsuccessful.

Accordingly, there is a need to provide ways of using multiple frequencyplans more efficiently.

SUMMARY

In a first principal aspect, the present invention provides a method ofselecting a sector-carrier, from among at least a first sector-carrierand a second sector-carrier, for transmitting a message. The firstsector-carrier has first downlink channels, including a first pilotchannel and a first paging channel, at a first downlink carrierfrequency, and first uplink channels, including a first access channel,at a first uplink carrier frequency. The second sector-carrier hassecond downlink channels, including a second pilot channel and a secondpaging channel, at a second downlink carrier frequency, and seconduplink channels, including a second access channel, at a second uplinkcarrier frequency. In accordance with the method, a mobile stationmeasures a first total received power at the first downlink carrierfrequency and a first pilot signal level of the first pilot channel. Themobile station measures a second total received power at the seconddownlink carrier frequency and a second pilot signal level of the secondpilot channel. The mobile station selects from among at least the firstsector-carrier and the second sector-carrier in accordance with aselection algorithm. The selection algorithm specifies a comparison ofthe first total received power to the second total received power and acomparison of the first pilot signal level to the second pilot signallevel.

In a second principal aspect, the present invention provides a method ofselecting a channel for a mobile station to use for transmitting amessage. In accordance with the method, the mobile station monitors acurrent paging channel that is associated with a current pilot channelat a current carrier frequency. At least one candidate carrier frequencydistinct from the current carrier frequency is identified. At least onecandidate pilot channel, including at least one pilot channel at the atleast one candidate carrier frequency, is identified. The mobile stationmeasures received signal strengths of the current carrier frequency andof the at least one candidate carrier frequency. The mobile station alsomeasures pilot signal levels of the current pilot channel and of the atleast one candidate pilot channel. The received signal strengths of thecurrent carrier frequency and the at least one candidate carrierfrequency are compared, and the pilot signal levels of the current pilotchannel and the at least one candidate pilot channel are compared. Acarrier frequency that has its received signal strength greater thanthat of the current frequency is selected, from among the at least onecandidate carrier frequency, as a selected carrier frequency. A pilotchannel, at the selected carrier frequency, that has its pilot signallevel greater than that of the current pilot channel is selected, fromamong the at least one candidate pilot channel, as a selected pilotchannel. The mobile station transmits the message in an access channelassociated with the selected pilot channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a wireless telecommunicationsnetwork, in accordance with an exemplary embodiment of the presentinvention;

FIG. 2 is a schematic diagram showing the wireless coverage areasprovided by the wireless telecommunications network of FIG. 1, inaccordance with an exemplary embodiment of the present invention;

FIG. 3 is a schematic diagram illustrating the downlink and uplinkchannels in two of the neighboring sectors in FIG. 2, in accordance withan exemplary embodiment of the present invention;

FIG. 4 is a flow chart illustrating a method for selecting asector-carrier to use to transmit a call origination message, inaccordance with an exemplary embodiment of the present invention; and

FIG. 5 is a flow chart illustrating a selection algorithm, in accordancewith an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 1. Overview

The present invention, in its exemplary embodiments, beneficiallyenables more efficient use of multiple frequencies by having a mobilestation monitor the RF conditions of multiple sector-carriers todetermine which sector-carrier may be optimal for transmitting amessage, such as a call origination message. The sector-carriers thatthe mobile station may monitor may include: (i) the mobile station'scurrent sector-carrier, i.e., the sector-carrier with which the mobilestation is associated while in an idle state; and (ii) one or moreother, candidate sector-carriers. The candidate sector-carriers mayinclude one or more sector-carriers that use different carrierfrequencies than the current sector-carrier. For example, a candidatesector-carrier may be associated with the same sector as the currentsector-carrier but use different carrier frequencies. The candidatesector-carriers may also include one or more sector-carriers that usethe same carrier frequencies as the current sector-carrier but havedistinct pilot channels. For example, a candidate sector-carrier may usethe same carrier frequencies as the current sector-carrier but may beassociated with a “neighboring” sector.

The RF conditions that the mobile station monitors may include the totalreceived power or received signal strength at a given carrier frequency(I₀), the absolute signal level (energy per chip) of a pilot channel(E_(c)), and/or the relative signal level of a pilot channel (E_(c)/I₀).The mobile station may then apply a selection algorithm to select asector-carrier based on these RF measurements. In accordance with theselection algorithm, the mobile station may compare the total receivedpower at the downlink carrier frequency of its current sector-carrierwith that of each of the candidate sector-carriers. The mobile stationmay also compare the pilot channel signal levels (absolute and/orrelative) of its current sector-carrier with that of each of thecandidate sector-carriers. If one of the candidate sector-carriers has atotal received power that is higher than that of the currentsector-carrier and has a pilot channel signal level (absolute and/orrelative) that is higher than that of the current sector-carrier, thenthe mobile station may select that candidate sector-carrier to transmitthe message. Otherwise, the mobile station may select its currentsector-carrier. The mobile station may then transmit the message in anaccess channel of the selected sector-carrier. The mobile station mayalso begin monitoring the paging channel of the selected sector-carrierfor messages from the network.

In many cases, the mobile station may select a sector-carrier in thisway in order to originate a call. For example, a user may dial digitscorresponding to a desired directory number and then actuate a “TALK” or“SEND” button on the mobile station. In response to this user input, themobile station may measure the RF conditions of the currentsector-carrier and one or more candidate sector-carriers and selecteither the current sector-carrier or a candidate sector-carrier that has“better” RF conditions according to the selection algorithm. Thus, inone example, a candidate sector-carrier may be deemed better if it hasboth a higher E_(c)/I_(o) and a higher I_(o) than the currentsector-carrier. In another example, a candidate sector-carrier may bedeemed better if it has E_(c)/I_(o), E_(c) and I_(o) that are all higherthan the current sector-carrier. Alternatively, other criteria could beused to select a sector-carrier.

The mobile station may then use an access channel of the selectedsector-carrier to transmit a call origination message to the network. Inresponse, the network may transmit a message to the mobile station,e.g., via the paging channel of the selected sector-carrier, that mayeither accept or deny the mobile station's request to use the selectedsector-carrier. If the network accepts the mobile station's selection,the network may instruct the mobile station to use particular trafficchannels allocated to the selected sector-carrier (e.g., a particularuplink traffic channel to transmit voice, data, or other media and aparticular downlink traffic channel to receive the voice, data, or othermedia for the call). However, if the network does not accept the mobilestation's request, the network may instruct the mobile station to useparticular traffic channels allocated to a non-selected sector-carrier.The non-selected sector-carrier could be, for example, the mobilestation's original, idle-state sector-carrier or a candidatesector-carrier that the mobile station did not select for the callorigination request.

By attempting to find what sector-carrier has optimal RF conditions, inaccordance with the exemplary embodiments described herein, a mobilestation may beneficially be able to originate calls or transmit othermessages more reliably.

2. Exemplary Architecture

Referring to the drawings, FIG. 1 is a simplified block diagram of anexemplary wireless telecommunications network 10 in which exemplaryembodiments of the present invention may be employed. Wirelesstelecommunications network 10 includes a plurality of base transceiverstations (BTSs) 12, 14, 16, 18, and 20, each of which provides awireless coverage area within which the BTS may communicate with one ormore mobile stations, such as mobile station 22, over an air interface.Mobile station 22 may be a wireless telephone, a wireless personaldigital assistant (PDA), a wirelessly equipped laptop computer, or otherwireless communication device. The communications between a BTS, such asBTS 12, and mobile station 22 may occur in a digital format, such asCDMA, TDMA, or GSM. Preferably, CDMA is used. Applicable CDMA formatsare described, for example, in TIA/EIA/IS-95-B (published Oct. 31, 1998)and in TIA/EIA/IS-2000 Series, Rev. A-2 (published April 2002), whichare incorporated herein by reference.

BTSs 12, 14, 16, 18, and 20 may be controlled by a base stationcontroller (BSC) 24, which, in turn, may be controlled by a mobileswitching center (MSC) 26. MSC 26 is connected to the public switchedtelephone network (PSTN) 28 and may use a signaling system, such as SS7,to route calls through PSTN 28. MSC 26 may also be able to signal to ahome location register (HLR) 30 and to a service control point (SCP) 32.This signaling may occur via one or more signal transfer points (STPs),such as, for example, STP 34. The signaling between MSC 26 and HLR 30may conform to IS-41 specifications. A recent revision of theIS-41-specifications, ANSI/TIA/EIA-41-D-97, published in December 1997,is incorporated herein by reference. The signaling between MSC 26 andSCP 32 may conform to the specification “Wireless Intelligent Network,”TIA/EIA/IS-771, published Aug. 1, 2001, which is incorporated herein byreference. Other signaling protocols could be used, however. In thisway, MSC 26, BSC 24, and BTS 12 may connect incoming calls from PSTN 28,which may originate from landline telephones, mobile stations, or othercommunication devices, to mobile stations in the wireless coverage areaof BTS 12, such as mobile station 22. Similarly, MSC 26, BSC 24, and BTS12 may connect calls originating from mobile station 22 to theirdestinations, via PSTN 28.

Although FIG. 1 shows BSC 24 connected to five BTSs, in general, BSC 24may be connected to a greater or fewer number of BTSs. In addition,although FIG. 1 shows BTSs 12, 14, 16, 18, 20 all connected to the sameBSC, in general, these BTSs could be connected to different BSCs.Similarly, although FIG. 1 shows MSC 26 connected to one BSC, ingeneral, MSC 26 may be connected to more than one BSC. Other variationsare possible as well.

Mobile station 22 may also be able to access a packet-switched network36, such as the Internet. For example, mobile station 22 may accesspacket-switched network 36 for e-mail, wireless web browsing, instantmessaging, or “push-to-talk” teleconferencing. To provide access topacket-switched network 36, BSC 24 may include a packet control function(PCF), and a packet data serving node (PDSN) 38 may connect BSC/PCF 24to packet-switched network 36. The communications between BSC/PCF 24,MSC 26, and PDSN 38 may conform to “third generation” (3G)specifications. Examples of such 3 G specifications include “Wireless IPNetwork Standard,” 3GPP2 P.S0001-A, dated Jul. 16, 2001 and “3GPP2Access Network Interfaces Interoperability Specification,” 3GPP2A.S0001-A, dated June 2001, which are incorporated herein by reference.

In an alternative approach for providing access to packet-switchednetwork 36, MSC 26 may be connected to packet-switched network 36 via aninterworking function (IWF) 40. MSC 26 and IWF 40 may allow mobilestations, such as mobile station 22, to access packet-switched network36 in circuit-switched data (CSD) sessions.

FIG. 2 schematically illustrates the wireless coverage areas provided byBTSs 12, 14, 16, 18, and 20 in accordance with an exemplary embodiment.Such wireless coverage areas may include cells and/or sectors. FIG. 2shows BTSs 12-20 located in cells 112-120, respectively. In addition, inthis exemplary embodiment, each BTS provides three sectors in each cell,such as by using directional antennas. Each sector shown in FIG. 2 isidentified by the number identifying the cell, with either an “a”, “b”,or “c” appended thereto. In this way, wireless telecommunicationsnetwork 10 provides sectors 112 a through 120 c, as shown in FIG. 2.

As will be apparent to those of ordinary skill in the art, FIG. 2illustrates wireless sectors 112 a through 120 c in only a highlyidealized fashion. In practice, the geographic extent of sectors 112 athrough 120 c may be very different than shown in FIG. 2. For example,while sectors 112 a through 120 c are generally contiguous, they mayoverlap to some extent, such that, in many locations, a mobile stationcan communicate in more than one sector. Thus, sectors 112 a through 120c shown in FIG. 2 are more representative of areas of optimal wirelesscommunication.

In the example shown in FIG. 2, mobile station 22 is associated withsector 112 c that is provided by BTS 12. However, because of theoverlapping wireless coverage described above, mobile station 22 mayalso be able to communicate using one or more neighboring sectors. Asdescribed in more detail below, mobile station 22 may have a neighborlist that identifies such neighboring sectors, e.g., by their pilotchannels. For example, with mobile station 22 located in sector 112 c, aneighbor list might define sectors 112 a, 112 b, 118 b, and 114 a asneighboring sectors. Mobile station 22 may obtain its neighbor list fromneighbor list messages or extended neighbor list messages transmitted bynetwork 10.

Each sector may have allocated to it a plurality of downlink channelsthat use a downlink carrier frequency and a plurality of uplink channelsthat use an uplink carrier frequency. The downlink channels mightinclude, for example, a pilot channel, a paging channel, and a pluralityof forward traffic channels. The uplink channels might include, forexample, a plurality of access channels and a plurality of reversetraffic channels. The various downlink channels and uplink channels maybe distinguished by different codes, in the case of CDMA operation, orby different time slots, in the case of TDMA or GSM operation.

Moreover, some or all of the sectors may use multiple frequency plans,i.e., multiple downlink carrier frequencies and multiple uplink carrierfrequencies. FIG. 3 schematically illustrates an example in whichsectors 112 c and 114 a use multiple frequency plans. As shown in FIG.3, sector 112 c, provided by BTS 12, includes two sector-carriers: (i)sector-carrier 200 that has downlink channels 202 and uplink channels204; and (ii) sector-carrier 206 that has downlink channels 208 anduplink channels 210. Thus, in this example, sector 112 c uses two setsof carrier frequencies. In particular, downlink channels 202, uplinkchannels 204, downlink channels 208, and uplink channels 210 may usecarrier frequencies f₁, f₂, f₃, and f₄, respectively.

As shown in FIG. 3, sector 114 a, provided by BTS 14, includes threesector-carriers: (i) sector-carrier 212 that has downlink channels 214and uplink channels 216; (ii) sector-carrier 218 that has downlinkchannels 220 and uplink channels 222; and (iii) sector-carrier 224 thathas downlink channels 226 and uplink channels 228. Thus, in thisexample, sector 114 a uses three sets of carrier frequencies. Inparticular, downlink channels 214, uplink channels 216, downlinkchannels 220, uplink channels 222, downlink channels 226, and uplinkchannels 228 may use carrier frequencies f₁, f₂, f₃, f₄, f₅, and f₆,respectively.

As this example illustrates, some of the downlink channels and uplinkchannels in neighboring sectors may use the same carrier frequencies.Thus, in the example illustrated in FIG. 3, sector-carriers 200 and 206in sector 112 c use the same carrier frequencies as sectors-carriers 212and 218, respectively, in sector 114 a. In CDMA systems, this may beaccomplished by the use of different codes in neighboring sectors.However, some sectors may use carrier frequencies that are not used inneighboring sectors. Thus, in the example illustrated in FIG. 3,sector-carrier 224 in sector 114 a uses carrier frequencies f₅ and f₆,which are not used in sector 112 c. This may occur, for example, whenBTS 14 is configured to use frequencies f₅ and f₆ but BTS 12 is not.

3. Exemplary Operation

An exemplary operation is illustrated in FIG. 4. In this example, themobile station selects a sector-carrier, in response to user input, touse to transmit a call origination message. It is to be understood,however, that the mobile station may follow a similar procedure toselect a sector-carrier to use to transmit other types of messages.Moreover, the mobile station may transmit such messages in response touser input, in response to instructions from the wireless network, orfor some other reason. For purposes of illustration, FIG. 4 assumes aCDMA air interface, i.e., different channels being identified bydifferent codes. However, a similar process could be applied to the casewhen different channels are distinguished on the basis of time slots, orin other ways.

With reference to FIG. 4, the process may begin with the mobile stationin an idle state and associated with a current sector-carrier, asindicated by block 300. For example, the mobile station may beassociated with sector-carrier 200 in sector 112 c, as shown in FIGS. 2and 3. In this idle state, the mobile station is not engaged in a calland is not using traffic channels of sector-carrier 200. However, themobile station may be associated with sector-carrier 200 in that themobile station is monitoring a paging channel of sector-carrier 200.Thus, the wireless network can reach mobile station, e.g., to notify themobile station of an incoming call or to provide other information orcommands to the mobile station, by transmitting messages over the pagingchannel of sector-carrier 200.

At some point, the user of the mobile station dials digits to originatea call, as indicated by block 302. In some cases, the user may place acall by interacting with a user interface of the mobile station. Theuser interface may include, for example, a keypad, buttons, a touchscreen, or other controls. Thus, to place a call, may enter digits usinga keypad and then actuate a “TALK” or “SEND” button. Alternatively, theuser may use voice-activated dialing to place a call by voicinginstructions to the mobile station, without physically contacting a userinterface.

In response to the user's instruction to originate a call, the mobilestation may identify one or more “candidate” sector-carriers, asindicated by block 304. A candidate sector-carrier has a downlinkcarrier frequency and/or a pilot channel code that is different fromthat of the mobile station's current sector-carrier. The mobile stationmay construct a list of candidate sector-carriers by identifyingcandidate carrier frequencies and candidate pilot channels. Thecandidate carrier frequencies may include the current carrier frequency(i.e., the downlink carrier frequency of the mobile station's currentsector-carrier) and at least one carrier frequency distinct from thecurrent carrier frequency. The candidate pilot channels may include: (i)pilot channels that have different codes than the current pilot channel(i.e., the pilot channel of the mobile station's currentsector-carrier); and (ii) pilot channels that have the same code as thecurrent pilot channel (i.e., in the same sector as the mobile station'scurrent sector-carrier) but use different carrier frequencies.

The mobile station may identify candidate carrier frequencies byreferring to a carrier frequency list and may identify candidate pilotchannels by referring to a neighbor list. The carrier frequency list mayspecify the carrier frequencies that the mobile station can use totransmit and receive, e.g., based on the mobile station's hardware,software, and/or authorizations. The carrier frequency list may beincluded in the software or firmware of the mobile station when it isprovided to the user. Alternatively, the list of useable carrierfrequencies may be provisioned into the mobile station at a later time,for example, as a result of communication with the wireless network.

With reference to FIG. 3, an exemplary carrier frequency list mightidentify frequencies f₁, f₃, and f₅ as the downlink carrier frequenciesthat the mobile station can receive and frequencies f₂, f₄, and f₆ asthe uplink carrier frequencies that the mobile station can transmit.From this list, the mobile station might select all of the identifieddownlink carrier frequencies as candidates, i.e., f₁, f₃, and f₅.Alternatively, the mobile station might identify only a subset ascandidate carrier frequencies. Although the carrier frequency list inthis example identifies three downlink carrier frequencies and threeuplink frequencies, it is to be understood that a carrier frequency listcould identify a greater or fewer number of carrier frequencies.

The neighbor list that the mobile station uses may be based on one ormore neighbor list messages and/or extended neighbor list messages thatthe wireless network previously transmitted to the mobile station. Theneighbor list may identify the pilot channels, such as by code, of“neighboring” sectors. In some cases, the neighbor list may specifycarrier frequencies as well, so as to identify “neighboring”sector-carriers. With reference to FIG. 2, when the mobile station is insector 112 c, an exemplary neighbor list might identify the pilotchannels (either with or without also specifying carrier frequencies)used in sectors 112 a, 112 b, 114 a, and 118 b. The mobile station maythen identify all of the pilot channels in the neighbor list ascandidates. Alternatively, the mobile station might identify only asubset as candidates. In some embodiments, the mobile station might notidentify any pilot channels from neighboring sectors as candidates, but,instead, identify only pilot channels in the mobile station's currentsector as candidates.

After identifying the candidate carrier frequencies and candidate pilotchannels, the mobile station may identify as a candidate sector-carrier:(i) any sector-carrier that is in its current sector (and, thus, has thesame pilot channel code) but uses a candidate carrier frequency distinctfrom the current carrier frequency; and (ii) any sector-carrier that hasa pilot channel code identified in the neighbor list and uses acandidate carrier frequency. Thus, even though the mobile station mayhave received from the wireless network an extended neighbor listmessage that specifies carrier frequencies of neighboringsector-carriers, the mobile station may not necessarily limit itself tothose carrier frequencies. The mobile station may also identify ascandidates neighboring sector-carriers that have other carrierfrequencies on the mobile station's carrier frequency list.

In this example, in which the mobile station is in sector 112 c andassociated with sector-carrier 200 with downlink carrier frequency f₁,the mobile station might identify candidate sector-carriers based ondownlink carrier frequencies f₁, f₃, and f₅ being identified in itscarrier frequency list and based on neighboring sectors 112 a, 112 b,114 a, and 118 b being identified as neighboring sectors in its neighborlist. Thus, with reference to FIG. 3, the mobile station may identify ascandidate sector-carriers: (i) sector-carrier 206 in sector 112 c; (ii)sector-carriers 212, 218, and 224 in sector 114 a; and (iii) one or moresector-carriers in each of sectors 112 a, 112 b, and 118 b.

The mobile station then makes RF measurements of the current andcandidate sector-carriers, as indicated by block 306. The RFmeasurements may include the total received power or received signalstrength at a given carrier frequency (I₀), the absolute signal level(energy per chip) of a pilot channel (E_(c)), and/or the relative signallevel of a pilot channel (E_(c)/I₀). For example, the mobile station maymeasure I₀ for the current carrier frequency and for all other candidatecarrier frequencies and may measure E_(c) for the current pilot channeland for all of the candidate pilot channels. The mobile station may thencalculate E_(c)/I₀ for the current pilot channel and for all of thecandidate pilot channels.

The mobile station then uses the results of these RF measurements toselect a sector-carrier in accordance with a selection algorithm, asindicated by block 308. As described in more detail below, an exemplaryselection algorithm may specify: (i) a comparison of I₀ for thecandidate carrier frequencies distinct from the current carrierfrequency to that of the current carrier frequency; and (ii) acomparison of the signal level (either in absolute terms, as in E_(c),or in relative terms, as in E_(c)/I₀) of the candidate pilot channels tothat of the current pilot channel. Thus, a candidate sector-carrier maybe selected if it has I₀ greater than that of the current carrierfrequency and has E_(c)/I₀ greater than that of the current pilotchannel. If no candidate sector-carrier meets these criteria, then themobile station may select the current sector-carrier.

After the sector-carrier is selected, the mobile station transmits thecall origination message in an access channel of the selectedsector-carrier, as indicated by block 310. The mobile station may thenreceive a channel assignment message from the wireless network inresponse to the call origination message, as indicated by block 312. Thechannel assignment message may indicate whether the wireless networkaccepts the mobile station's selected sector-carrier. In particular, thechannel assignment message may instruct the mobile station to usetraffic channels that are in the selected sector-carrier, therebyindicating the network's acceptance of the mobile station's selection.In some cases, however, the wireless network may not accept the mobilestation's selection, e.g., because of a lack of available trafficchannels in that sector-carrier, load balancing considerations, or forsome other reason. In such cases, the channel assignment message mayinstruct the mobile station to use traffic channels in a sector-carrierthat the mobile station did not select for the call origination message.Whether or not the network accepts the mobile station's sector-carrierselection, the mobile station proceeds by using the traffic channelsthat the network assigned for the call, as indicated by block 314.

FIG. 5 illustrates an exemplary selection algorithm, which maycorrespond to block 308 in FIG. 4. In this example, the comparisonbetween the current sector-carrier and the candidate sector-carriers ismade based on I₀ and E_(c)/I₀. However, it is to be understood, that thecomparison could alternatively be made based on I₀ and E_(c), based onI₀, E_(c)/I₀, and E_(c), or based on other types of RF measurements.

With reference to FIG. 5, the selection algorithm may first determinewhether any candidate carrier frequency was found to have I₀ greaterthan that of the current carrier frequency (the carrier frequency of themobile station's current sector-carrier), as indicated by block 400. Ifno candidate carrier frequency meets this criterion, then the mobilestation may select the current sector-carrier, as indicated by block402. If, however, one or more candidate carrier frequencies do have I₀greater than that of the current carrier frequency, then the mobilestation may select the candidate carrier frequency with the highest I₀,as indicated by block 404. In this example, a candidate carrierfrequency is required to have I₀ that is simply greater than that of thecurrent carrier frequency in order to be selected. Alternatively, theselection algorithm could require I₀ for the candidate carrier frequencyto be greater than I₀ of the current carrier frequency by at least apredetermined amount in order to be selected.

Once a candidate carrier frequency is selected, the mobile station maythen identify those candidate pilot channels that use this selectedcarrier frequency, as indicated by block 406. The selection algorithmmay then determine whether any of those identified pilot channels (i.e.,pilot channels that use the selected carrier frequency) has an E_(c)/I₀that is greater than that of the current pilot channel, as indicated byblock 408. If none of the identified pilot channels meets thiscriterion, then the current sector-carrier is selected, as indicated byblock 410. If, however, one or more identified pilot channels do haveE_(c)/I₀ greater than that of the current pilot channel (the pilotchannel of the current sector-carrier), then the mobile station mayselect the identified pilot channel that has the highest E_(c)/I₀, asindicated by block 412. Alternatively, instead of requiring a pilotchannel's E_(c)/I₀ to be simply greater than that of the current pilotchannel to be selected, the selection algorithm could require a pilotchannel's E_(c)/I₀ to be greater than that of the current pilot channelby at least a predetermined amount in order to be selected.

4. Conclusion

Exemplary embodiments of the present invention have been describedabove. Those skilled in the art will understand, however, that changesand modifications may be made to these embodiments without departingfrom the true scope and spirit of the invention, which is defined by theclaims.

1. A method for a mobile station, said method comprising: said mobilestation monitoring a current paging channel that is associated with acurrent pilot channel at a current carrier frequency; said mobilestation receiving input from a user; and in response to said input, saidmobile station: identifying at least one candidate carrier frequencydistinct from said current carrier frequency; identifying at least onecandidate pilot channel, wherein said at least one candidate pilotchannel includes at least one pilot channel at said at least onecandidate carrier frequency; measuring received signal strengths of saidcurrent carrier frequency and of said at least one candidate carrierfrequency; measuring pilot signal levels of said current pilot channeland of said at least one candidate pilot channel; comparing saidreceived signal strengths of said current carrier frequency and said atleast one candidate carrier frequency; comparing said pilot signallevels of said current pilot channel and said at least one candidatepilot channel; selecting a selected carrier frequency, from among saidat least one candidate frequency, that has its received signal strengthgreater than that of said current carrier frequency; selecting aselected pilot channel at said selected carrier frequency, from amongsaid at least one candidate pilot channel, that has its pilot signallevel greater than that of said current pilot channel; and transmittinga message in an access channel associated with said selected pilotchannel.
 2. The method of claim 1, wherein said pilot signal levels areabsolute signal levels.
 3. The method of claim 1, wherein said pilotsignal levels are relative signal levels.
 4. The method of claim 1,wherein identifying at least one candidate carrier frequency distinctfrom said current carrier frequency comprises said mobile stationreferring to a carrier frequency list.
 5. The method of claim 1, whereinidentifying at least one candidate pilot channel comprises said mobilestation referring to a neighbor list.
 6. The method of claim 1, whereinsaid input comprises an instruction to originate a call.
 7. The methodof claim 6, wherein said message is a call origination message.